As the 3GPP evolves UMTS to a pure packet switched technology in 3GPP Long Term Evolution (LTE), the way voice traffic will be carried is in the form of Voice over IP (VoIP). The current support of voice over IP (VoIP) in 3GPP LTE is tailored to the adaptive multirate (AMR) vocoder which has been used in GSM and UMTS. For CDMA operators migrating to LTE, enhanced variable rate codec (EVRC) is the vocoder of choice, given that it is currently used in their CDMA network and they wish to avoid the delay and quality degradation which would occur if they had to transcode EVRC to AMR when they migrate to LTE (that is, anytime an LTE user on their network called an EV-DO user or 1x user, transcoding would have to occur between the AMR codec on the LTE network and the EVRC codec on the 1x and EV-DO networks).
Unlike the AMR vocoder, the EVRC vocoder generates variable sized packets during a talk spurt, and the improved EVRC-B vocoder actually allows different modes which can utilize smaller packets more often in order to improve capacity on CDMA networks while trading off some quality in the speech.
Unfortunately, the current LTE uplink support for VoIP only allows the UE to transmit with a single packet size, which has to be large enough in order to accommodate the largest VoIP packet size. This is fine for AMR vocoders which output just one packet size during a talk spurt (for example 244 bits for AMR 12.2 kbps), however it becomes quite inefficient for the EVRC vocoder which outputs a variety of packet sizes during a talk spurt (as large as 171 bits and as small as 40 bits). This results in significant padding overhead using the current 3GPP LTE standard, which in turn results in higher than needed UE transmit power level that generates extra interference to other cells, and in the end reduces overall system capacity.